Well cementing method employing an oil base preflush

ABSTRACT

This specification discloses a method of cementing casing in a well having an oil base drilling mud therein. A preflush solution comprised of an oil and a dispersant, which preflush solution has a lower viscosity and a lower gel strength than the drilling mud in the well, is flowed through the annular space between casing positioned in the well and the well of the well. Thereafter, a cement slurry is flowed into the annular space and allowed to set.

United States Patent Messenger [is] 1 3,688,845 1 1 Sept.'5, 1972 [541 3WELL CEMENTING METHOD EMPLOYING AN 01L BASE PREFLUSH [72] Inventor:Joseph U. Mesenger, Dallas, Tex. [7 3] Assignee: Mobil Oil Corporation,2

[22] Filed: March 8, 1971 2 11 Appl. No.: 122,123

[52] US. Cl. ..166/291, 166/292, 166/312 [51] Int. Cl. ..E21b 21/00,E2lb 33/14 [58] Field 0i Search ..166/291, 285, 292, 293, 294,

[56] References Cited UNITED STATES PATENTS 2,848,051 8/1958 Williams..166/291 3,022,823 2/ 1962 Caldwell et a1. ..166/291 3,291,21 1 12/1966Savins et a1 166/285 3,299,952 1/1967 Savins et al ..166/285 3,342,2649/ 1967 Willard, Sr. 166/300 3 348,612 10/ 1967 Messenger 166/2923,411,580 11/1968 McKinney et al ..166/312 X 3,467,193 9/ 1969 Messenger166/292 3,532,168 10/1970 Webb ..166/292 3,625,286 12/ 1971 Parker166/291 Primary Examiner-Stephen .l. Novosad Attorney-William J.Scherback, William D. Jackson and Henry L. Ehrlich ABSTRACT 12 Claims,No Drawings WELL CEMENTING METHOD EMPLOYING AN OIL BASE PREFLUSI-IBACKGROUND OF THE INVENTION subterranean formations for the purpose ofproducing l hydrocarbons or water therefrom, or for injecting fluidsinto the subterranean formation.

Rotary drilling techniques wherein a drilling fluid or mud is circulatedbetween the surface of the earth and the bottom of the well are commonlyemployed in drilling wells. Oil base muds, including water-in-oilemulsion muds of low water content, wherein oil forms the continuousphase, are often used in the drilling and completion of wells. Drillingmuds serve many useful purposes, including removal of earth cuttingsfrom the well, control of formation pressures, and cooling andlubricating of the drill bit used in forming the well. However, thereare also certain detrimental characteristics associated with drillingmuds. For example, drilling muds tend to flow from the well into exposedpermeable formations with the result that mud solids are filtered out onthe wall of the well, forming a filter cake thereon. Thick filter cakesare particularly harmful in that they may impede or stop the movement ofdrilling tools in the well. Even thin filter cakes are detrimental inthe completion of wells in that they interfere with obtaining a goodcement bond between the wall of the well and the casing positioned inthe well.

Drilling muds may be considered to be non-Newtonian fluids and arereferred to as plastic-type fluids. Their fluid movements are known asplastic flow. Two flow properties, plastic viscosity and yield value,must be known in order to describe the flow behavior of a drilling mudas compared with one flow property, viscosity, required to define theflow characteristics of a Newtonian fluid. Another property often usedin characterizing a drilling mud at rest is gel strength.

Plastic viscosity is that part of flow resistance in a mud whichrepresents mechanical friction 1) between solids in the mud, (2) betweenthe solids and the liquid which surround them, and (3) of the shearingof the liquid. Yield value, the second component of resistance to flowin a drilling fluid, is a measurement of the interparticle forces in themud. These forces are the result of positive and negative electricalcharges located on or near particle surfaces. If the mud is allowed tocome to rest, the forces tend to move the solid particles around untilthey reach a geometrical arrangement where the attraction and repulsionforces are best satisfied. A gelstrength measurement is an indication ofthe magnitude of these forces under conditions of rest. Yield value is ameasurement under flowing conditions of those forces in the mud whichcause gel strength under rest conditions.

Various procedures and apparatus, including the Stormer viscosimeter andthe Fann V-G Meter, have been used in determining plastic viscosity,yield value, and gel strengths of drilling muds. In a paper entitledMeasurement of Plastic Flow Properties by Fred Chisholm and Sam Kohen,PETROLEUM EN- GINEER, vol. 26, No. 4, pp. B-87-B-90, April, 1954,

there is described a procedure employing a Model 30 Fann V-G Meter tomeasure the plastic viscosity, yield value, and gel strength of adrilling mud.

In the completion of wells, casing is normally lowered into the well anda cement slurry is flowed down the well and up the annular space formedbetween he casing and the wall of the well. Cement slurries have beenemplaced in the annular space between the casing and the wall of thewell by flowing them thereinto in direct contact with the mud, therebydisplacing the mud from the annular space. This procedure may result inpoor cement jobs because of contamination of the cement slurry andbecause of failure to remove the filter cake from the wall of the well.In order to obtain improved cement jobs, various techniques, includingthe use of preflush solutions or washes, have been developed. Forexample, Joseph G. Savins, in US. Pat. No. 3,291,211, describes amethod' of cementing a well wherein the drilling mud in the annularspace between casing and a wellbore wall is removed by passing into awell, prior to placing a cement slurry therein, a liquid comprising asolvent containing dissolved therein a solute which imparts viscoelasticproperties to the liquid. Joseph U. Messenger, in US. Pat. No.3,348,612, describes a technique of cementing casing in a well wherein aslug of liquid having mutual solubility for oil and water is passed intoa well and into the annular space formed between the wall ofthe well andthe outer surface of the casing and is followed by a cement slurry. Aslug of an organic solvent for oil and asphalt may be passed into theannular space ahead of the slug of liquid having mutual solubility foroil and water.

Another technique for cementing casing in a well is described by JosephU. Messenger in US. Pat. No. 3,467,193. A preflush comprised offormation water containing dispersants is flowed through the annularspace formed between casing and the wall of a well. Thereafter, aspearhead of cement slurry containing a turbulence inducer and silica ordiatomaceous earth particles and having a density at least as great asthe drilling mud employed in drilling the well is flowed into theannular space. This is followed by a densified cement slurry and thecement slurries allowed to set.

SUMMARY OF THE INVENTION This inventionis directed to a method ofcementing casing in a well penetrating a subterranean formation, whichwell has therein an oil base drilling mud. A slug of a preflush solutioncomprised of an oil and a dispersant, which solution has a lowerviscosity and lower gel strength than the drilling mud and hasa'critical turbulence pump rate of no greater than three barrels perminute, is flowed into the annular space between the casing and the wallof the well. Thereafter, cement slurry is emplaced in the annular spaceand allowed to set and form an impermeable cement sheath which bonds thecasing to the wall of the well. In another embodi-' ment, the preflushsolution further includes a viscosifier, and a weighting material.

DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention is directed to amethod of cementing casing in a well that has an oil base drilling mudtherein and has a filter cake on the wall of the well.

In accordance with this invention a preflush solution comprised of anoil and a dispersing agent is injected into a well that has an oil basedrilling mud therein and into the annular space between the casing andthe wall of the well to displace the mud and remove the filter cake fromat least a portion of the annular space. This is followed by a cementslurry which is emplaced in the annular space and allowed to set,thereby forming an impermeable cement sheath which bonds the casing tothe wall of the well. In another embodiment, water having a waterwetting surfactant therein is flowed into the annular space followingthe preflush solution and preceding the cement slurry to displace theoil from the surface of the casing and the wall of the well.

The preflush solution is designed for use in a well having an oil basedrilling mud therein and is formulated to have a lower viscosity andlower gel strength than the drilling mud. Further, the preflush solutionis formulated to have a critical turbulence pump rate of no greater thanthree barrels per minute. By the term critical turbulence pump rate ismeant the pump rate at which the preflush solution will go intoturbulence when flowed through a rfi-inch annulus formed between twoconcentric pipes, the smaller of which has an outside diameter of 7inches, and the larger of which has an inside diameter of 8 inches. Alow critical turbulence pump rate is beneficial for two reasons: l) thewell wall is exposed to turbulent action a maximum length of time for agiven volume of preflush solution; and (2) a lower pump pressure can beused to obtain turbulent flow than must be used for solutions havinghigher critical turbulence pump rates. The dispersing agent employed incarrying out the present invention may be any suitable material whichfunctions to promote and maintain separation of the individual,extremely fine particles of solids which are usually of colloidal size.As will be recognized by those skilled in he art, various suchdispersing agents are conventionally employed in drilling muds in orderto maintain adequate dispersion of clays added to impart desiredrheological properties to such muds. A particular dispersing agent whichgives good results when employed in the preflush and which is preferredin practicing the present invention is lecithin. Lecithin is a naturallyoccurring amino, and is available from Baroid Division, National LeadCompany, under the trade name Driltreat.

A particular preflush solution found to give good results is comprisedof an oil and lecithin in an amount of l to pounds per barrel ofpreflush solution. Oils that are preferred are kerosene, diesel oil, andmixtures thereof because they are compatible with oils used in oil basedrilling muds. However, other oils such as light crude oils may be used.

This preflush solution goes into turbulence at a low rate of flow, thusexposing the wall of the well to the scouring action of turbulent flowfor a maximum amount of time for a given size slug of solution. Inaddition, the dispersing agent permeates the filter cake and throughdispersing action causes a deterioration of the filter cake. Thus, bycombination of the turbulent action and dispersing action the filtercake is effectively removed. In addition, the turbulent action of thepreflush solution effectively displaces the drilling mud from theportion of the annular space into which cement slurry is to be emplaced.A l0- to SO-barrel slug of preflush solution injected into the well andinto the annular space preceding the cement slurry is sufficient todisplace the drilling mud and filter cake from the portion of theannular space of concern, thereby allowing a good cement sheath to beformed between the casing and the wall of the well.

An aqueous wash containing a water wetting surfactant may be flowed intothe annular space subsequent to the preflush solution and prior to thecement slurry to remove from the casing the oil film remaining from thepreflush solution, thereby providing a clean surface and enhancing thebond formed between the cement sheath and the casing. This aqueous wash,when employed, should be flowed into the annular space in an amount offive to 50 barrels, preferably 10 to 20 barrels, and should contain 9%to 10 pounds of water wetting surfactant per barrel of water andpreferably 2 to 4 pounds per barrel. A suitable water wetting surfactantmay be chosen from the classes of nonionic and anionic surfactants. Anexample of a class of nonionic surfactants from which a suitable waterwetting surfactant may be chosen is the polyoxyethylene-aryl ethers ofthe structural formula:

wherein:

R is a hydrogen atom or an aliphatic substituent having no more thannine carbon atoms, and

n is an integer within the range of 10 to 100.

Within the class of nonionic surfactants a preferred surfactant is thepoloyoxyethylene ether of phenol having 30 ethylene oxide groups.Anionic surfactants which may be employed in the present invention maybe chosen form he class of salts of fatty acids or salts of sulfonicacid derivatives. Two examples of anionic surfactants are di-ethylsodium succinate and sodium lauryl sulfonate.

When cementing a well having a weighted oil base drilling mud therein, 1have found it desirable to displace the oil base drilling mud ahead ofthe cement slurry with a weighted preflush solution. It is particularlyimportant to use a weighted preflush solution having approximately thesame weight as the drilling mud when there exists in communication withthe well a formation having fluids under high pressure containedtherein. The use of such a preflush solution ensures that thehydrostatic pressure exerted by the drilling mud in the well is notlessened, thereby allowing the fluids under high pressure in theformation to flow uncontrollably into the well.

A weighted preflush solution which I have found to be particularlysuitable for such an application is comprised of an oil, a viscosifier,a weighting material, and a dispersing agent formulated to have a lowerviscosity and a lower gel strength than the drilling mud and to have acritical turbulence pump rate of no greater than three barrels perminute. Oils found to be particularly useful in this preflush solutionare kerosene, diesel oil and mixtures thereof, though other oils may beused. A

viscosifier found to be particularly useful is an amine substitutedbentonite used in a sufficient amount to increase the viscosity of thepreflush solution to a value which will prevent settling of theweighting material.

Normally no more than 5 pounds of viscosifier per barrel of preflushsolution are required to provide this viscosity. Such a viscosifier canbe prepared by treating bentonite, sodium form, with a long-chainquaternary ammonium salt and is available under the trade name ofGeltone." A weighting material suitable for use in v this preflushsolution is barite. A sufficient amount of barite should be used toincrease the weight of the preflush solution to a weight of at least 14pounds per gallon and preferably to approximately the weight of thedrilling mud in the well. A particular dispersing agent found to beuseful in this preflush solution is lecithin which is available underthe trade name of Driltreat. A sufficient amount of the dispersing agentis added to ensure that the weighted preflush solution has a criticalturbulence pump rate of no greater than three barrels per minute.Normally this requires no more than 5 pounds of dispersing agent perbarrel of preflush solution. The dispersing agent reduces theinterparticle forces in the preflush solution and thereby the yieldvalue of the preflush solution, thus enabling the solution to go intoturbulence at a low rate of flow.

What is claimed is:

1. A method of cementing casing in a well penetrating a subterraneanformation, said well having an oil base drilling mud therein,comprising:

a. flowing a slug of preflush solution comprised of an oil and adispersing agent, said solution having a lower viscosity and lower gelstrength than said drilling mud and having a critical turbulence pumprate of no greater than three barrels per minute, into the annular spacebetween said casing and the wall of said well;

b. flowing into said annular space and emplacing therein a cementslurry; and

c. allowing said cement slurry to set and form an impermeable cementsheath which bonds said casing to said wall of said well.

2. The method of claim 1 wherein said oil is selected from the groupconsisting of kerosene, diesel oil, and mixtures thereof.

3. The method of claim 1 wherein the volume of said slug of preflushsolution is within the range of to 50 barrels.

4. The method of claim 1 wherein said dispersing agent is lecithin.

5. The method of claim 4 wherein the amount of lecithin in said preflushsolution is l to 5 pounds per barrel of said preflush solution.

6. The method of claim 1 further comprising, subsequent to step (a) andprior to step (b), flowing a slug of an aqueous solution having a waterwetting surfactant therein into said annular space between said casingthe the wall of said well.

7. The method of claim 6 wherein said slug of an aqueous solution isinjected in an amount within the range of 5 to 50 barrels and containstherein to 10 pounds per barrel of a poIyoxyethylene-aryl ether of thestructural formula:

R is a hydrogen atom or an aliphatic substituent having no more thannine carbon atoms, and n is an integer within the range of 10 to 100.

8. A method of cementing casing in a well penetrating a subterraneanformation, said well having an oil base drilling mud therein,comprising:

a. flowing a slug of preflush solution comprised of an oil, a dispersingagent, a viscosifier, and a weighting material, said solution having alower viscosity and lower gel strength than said drilling mud and havinga critical turbulence pump rate of no greater than three barrels perminute, into the annular space between said casing and the wall of saidwell;

b. flowing through said annular space and emplacing therein a cementslurry; and

0. allowing said cement slurry to set and form an impermeable cementsheath which bonds said casing to said wall of said well.

9. The method of claim 8 wherein the volume of said slug of preflushsolution is within the range of five to 50 barrels.

10. The method of claim 9 wherein said oil is selected from the groupconsisting of diesel oil, kerosene, and mixtures thereof.

11. The method of claim 10 wherein said dispersing agent .is lecithin.

12. The method of claim 11 wherein said viscosifier is an aminesubstituted bentonite.

2. The method of claim 1 wherein said oil is selected from the groupconsisting of kerosene, diesel oil, and mixtures thereof.
 3. The methodof claim 1 wherein the volume of said slug of preflush solution iswithin the range of 10 to 50 barrels.
 4. The method of claim 1 whereinsaid dispersing agent is lecithin.
 5. The method of claim 4 wherein theamount of lecithin in said preflush solution is 1 to 5 pounds per barrelof said preflush solution.
 6. The method of claim 1 further comprising,subsequent to step (a) and prior to step (b), flowing a slug of anaqueous solution having a water wetting surfactant therein into saidannular space between said casing the the wall of said well.
 7. Themethod of claim 6 wherein said slug of an aqueous solution is injectedin an amount within the range of 5 to 50 barrels and contains therein1/2 to 10 pounds per barrel of a polyoxyethylene-aryl ether of thestructural formula: wherein: R is a hydrogen atom or an aliphaticsubstituent having no more than nine carbon atoms, and n is an integerwithin the range of 10 to
 100. 8. A method of cementing casing in a wellpenetrating a subterranean formation, said well having an oil basedrilling mud therein, comprising: a. flowing a slug of preflush solutioncomprised of an oil, a dispersing agent, a viscosifier, and a weightingmaterial, said solution having a lower viscosity and lower gel strengththan said drilling mud and having a critical turbulence pump rate of nogreater than three barrels per minute, into the annular space betweensaid casing and the wall of said well; b. flowing through said annularspace and emplacing therein a cement slurry; and c. allowing said cementslurry to set and form an impermeable cement sheath which bonds saidcasing to said wall of said well.
 9. The method of claim 8 wherein thevolume of said slug of preflush solution is within the range of five to50 barrels.
 10. The method of claim 9 wherein said oil is selected fromthe group consisting of diesel oil, kerosene, and mixtures thereof. 11.The method of claim 10 wherein said dispersing agent is lecithin. 12.The method of claim 11 wherein said viscosifier is an amine substitutedbentonite.